1. Field of the Invention
The present invention relates to a chemical vapor deposition method and apparatus for forming various deposited films such as metal films, semiconductor films and insulating films used in memory devices such as semiconductor devices and optical magnetic disks or in flat panel display devices. More particularly it relates to a chemical vapor deposition method and apparatus making use of a liquid starting material.
2. Related Background Art
Deposited films formed by chemical vapor deposition method (CVD method) using appropriate apparatus therefor (CVD apparatus) can be roughly grouped into metal films, semiconductor films and insulating films.
In the case of semiconductor films, a film forming method capable of obtaining uniform films with fewer faults is desired. As for insulating films, uniform films are also desired and a film forming method that can achieve excellent coating properties on step portions is desired. This is because most insulating films are used to insulate wirings from each other in an integrated circuit (IC) or to protect its uneven surface.
In case of the metal films, a film forming method that can achieve excellent uniformity and coating performance on step portions is also desired as in the case of the insulating films. This is because the metal films are mostly used in wiring materials for ICs. In such instances the coating properties on step portions at the holes thereof are required so that upper and lower wirings can be connected via openings called contact holes or through holes.
FIG. 1 diagrammatically illustrates the prior art CVD apparatus used for a prior art CVD method.
In FIG. 1, reference numeral 403 denotes a reaction chamber formed of quartz or the like, provided with substrate holders 410 that are disposed in plurality to support thereon a corresponding number of substrates 409 on which films are to be formed. Reference numeral 408 denotes an exhaust pipe, which is connected to a main pump 404 comprised of a mechanical booster pump and an auxiliary pump 405 comprised of a rotary pump. The pumps are used to evacuate the inside of the reaction chamber 403.
As for a gas feeding system, a bomb (bubbler) 402 having a bubbling mechanism that bubbles a liquid starting material, a gas pipe 406 through which carrier gas for the bubbling is fed, a valve 401, and a gas pipe 407 through which vaporized starting materials are fed into the reaction chamber 403 are provided.
Such a conventional CVD apparatus can be used so long as routine film formation is carried out, but it is sometimes unsuitable to provide fine structure or to form large area films as recently demanded. In other words, conventional apparatus are poor in multi-purpose uses, i.e., adaptability to any CVD methods. This problem will be discussed below by giving an example.
Recently, as materials for the wiring of highly integrated semiconductor devices called VLSI or ULSI, aluminum films formed not by sputtering but by CVD have attracted notice. In particular, in CVD methods making use of an organic compound comprising an organoaluminum compound, it is reported that conditions for deposition differ greatly between an insulator and a conductor. Hence, selective deposition can be made where aluminum is deposited only on the conductor or a semiconductor. This selective deposition of aluminum is very useful when fine integrated circuits are fabricated. In particular, when the ratio of depth to diameter (aspect ratio) of a hole is more than 1, the selective deposition can provide aluminum wiring that can not be realized by sputtering, which is a substitute technique. In sputtering, disconnection occurs when the hole has a large aspect ratio. The reason therefor will be explained below with reference to FIGS. 2A to 2C. In FIGS. 2A and 2B, reference numeral 201 denotes a monocrystalline silicon substrate; 202, an insulating film such as silicon dioxide film; and 203, a wiring material such as aluminum.
FIG. 2A illustrates how the wiring is formed when the hole has a small aspect ratio, and FIG. 2B how the wiring is formed when the hole has a large aspect ratio of 1 or more.
In sputtering, a hollow 204 or a void 205 is formed. On the other hand, in selective deposition by CVD, the hole is completely filled with aluminum 303 as shown in FIG. 2C, and there is a very low probability of disconnection.
In FIG. 3, reference numeral 301 denotes a silicon substrate; 302, an insulating film such as silicon dioxide film; 303, a metallic material such as aluminum deposited by CVD; and 304, wiring of aluminum deposited by sputtering or CVD.
Thus, in the case when the wiring of a fine semiconductor device is fabricated using the CVD apparatus shown in FIG. 1, a carrier gas CGS such as hydrogen, whose pressure is reduced by means of a reducing valve 401, is fed to the bubbler 402. Most of material gases that enable the selective deposition of aluminum are liquid at room temperature, as exemplified by dimethylaluminum hydride (DMAH) and triisobutylaluminum (TIBA). For this reason, the bubbling, i.e. the step of generating bubbles in the bubbler 402 is carried out, so that a mixed gas comprised of the carrier gas and saturated vapor of organoaluminum compound such as DMAH is fed into the reaction chamber. The mixed gas is thermally decomposed on the heated semiconductor substrates 409, and aluminum is deposited on the substrate as a result of its surface reaction with the substrates.
Unreacted gas in the reaction chamber 403 is exhausted outside by means of the main pump 404 and auxiliary pump 405.
However, difficulties have been encountered in scaling-up an experimental CVD apparatus that has stably achieved selective deposition to a mass-production CVD apparatus
This results in an increase in faults not only in the case of the metal films but also in the case of the semiconductor films, and results in a lowering of step portion coating properties in the case of the insulating films.
According to a finding made by the present inventors, the poor general-purpose properties are caused by the constitution of the conventional CVD apparatus, as follows.
First, the mixing ratio of the starting material liquid compound and other gas can only be very poorly controlled.
Second, a temperature change in the vicinity of the bubbler causes a change in the mixing ratio of the compound.
Third, residual gases in the bubbler cause a change in the mixing ratio of the compounds.